Method for the production of a bladder accumulator and bladder accumulator produced according to said method

ABSTRACT

The invention relates to a method for the production of a bladder accumulator ( 10 ) which separates two media chambers ( 16, 18 ) from one another in a storage housing ( 12 ) by means of a bladder body ( 14 ), comprising at least the following production steps:—extruding a plastic tube over the bladder body ( 14 );—shaping the plastic tube with the integrated bladder body ( 14 ) in a molding tool that corresponds to a predeterminable plastic core container ( 20 ), and—winding at least one plastic fiber from the outside on the plastic core container ( 20 ) for the purpose of creating the storage housing ( 12 ).

The invention concerns a method for the manufacture of a bladder accumulator, which separates two media chambers from each other in an accumulator housing by means of a bladder body. The invention concerns, moreover, a bladder accumulator manufactured according to said method.

The document DE 10 2006 004 120 A1 discloses a hydraulic accumulator, in particular in form of a bladder accumulator, for the purpose of accepting at least one fluid medium with a pressure vessel comprising a first synthetic sheath as well as a second synthetic sheath that at least partially surrounds said first synthetic sheath, wherein the first synthetic sheath is provided at least at its one end with a collar portion, which comprises an opening for a valve that is provided for controlling the media supply and discharge and wherein the collar portion and the second synthetic sheath are supported on an in-between-located outer support ring, which tapers in a wedge-like manner in the direction of a slot-shaped opening between said sheaths.

Due to the fact that in the known solution the slot-shaped opening between the sheaths is extended to a point where the sheaths are coaxially in contact with each other, and that in the opening a poppet valve is disposed for controlling the media supply and discharge, a sealed accumulator arrangement is provided that can be realised at relatively low manufacturing costs. Because the support extends via the wedge-like tapered outer support ring into the outer circumference section of both sheaths, any relative movements that may occur between the synthetic sheaths are absorbed by the outer support ring, and damaging delamination processes are directly avoided in this manner between the sensitive synthetic materials.

The technical terminology for the first synthetic sheath is also “synthetic core container” or “liner”. It is preferably made from polyamide by means of a blow moulding process or through rotational moulding. At the outer circumference said first synthetic core container or liner is reinforced through externally wound fibre windings, which constitutes the second synthetic sheath. To protect the fibre windings it is possible to embed the individual fibres in a basic matrix of duromers, for example epoxy resins, phenolic resins or in thermoplastic resins, for example in form of PA6, PA12, PP etc. In order to place the poppet valve, i.e. the valve for liquids, on the one side of the accumulator housing as well as the gas supply valve on the other opposite side of the accumulator housing, relatively large openings are required in the two-sheath accumulator housing, into which also the respective collar portions must be placed in order to secure the respective valve at its housing opening side.

Moreover, document DE 10 2011 111 098 A1 discloses a pressure vessel, which is preferably used for bladder accumulators, comprising a multi-part container body that consists of a tube-like centre section that provides at least on one of its two ends an end section, wherein a cover, which forms at least in part the respective end section, overlaps the centre section at the edge at least in the section of its respective end and forms a solid connection.

The known solution proposes to form the tube-like centre section as well as the respective cap from a fibre-reinforced composite material and/or from a fibre laminate, preferably from a glass fibre reinforced plastic (GRP). It is preferable to use standard GRP pipes for the centre section, wherein the caps for closing off said centre section are preferably made by way of a manual lamination process and may consist of comparable synthetic materials.

At the cap end of the accumulator housing, which will later accommodate the valve for the liquid, a relatively large opening is provided, which permits the accumulator bladder as the bladder body to be drawn, together with the gas filling valve that is attached through vulcanising, into the inside of the accumulator housing so that said gas valve is located on the opposite side of the housing in the respective opening in the accumulator housing that has a comparatively smaller diameter. With regard to the different sizes of the valve openings in the accumulator housing, the subsequently applied windings are different by necessity, wherein each individual winding will have a different formation when subsequently combined, in particular with regard to its wall diameter, so that, although the composite fibre layers can be simply wound, when forces are applied weak points become apparent, in particular when it is desired to achieve high strength values in axial direction of the accumulator housing.

Based upon said prior art it is the object of the invention to provide, in contrast, a method for the manufacture of a bladder accumulator that can be made cost-effectively and exhibits high strength values in the accumulator housing. This object is met by a method with the characteristics of claim 1 in its entirety. Moreover, the object is also to provide a bladder accumulator with the characteristics of claim 10 that is manufactured according to this method.

The method according to the invention for the manufacture of a bladder accumulator is characterized by the following production steps:

-   -   extrusion of a plastic tube over a bladder body;     -   moulding of the plastic tube with integrated bladder body in a         moulding tool that corresponds to a predeterminable synthetic         core container, and     -   winding at least one plastic fibre from the outside onto the         synthetic core container for the purpose of creating the         accumulator housing.

The solution according to the invention is based upon the idea of integrating the bladder body already during the moulding process of the synthetic core container or liner into said liner. To this end the plastic tube is extruded, preferably using the blow moulding process, over the bladder body and a blow mould, which usually consists of a moulding tool that is adapted to the shape of the synthetic core container, is closed with multiple parts, preferably two halves, of a mould.

Since the bladder body is now already integrated into the synthetic core container, which to that extent constitutes part of the overall accumulator housing of the bladder accumulator, the separating bladder or the bladder body respectively together with any gas filling valve bonded through vulcanising is no longer required to be pulled through fluid-passing openings to create the bladder accumulator, so that it is possible to choose to a large extent what size the supply or media openings should be. Insofar as the free access openings in the accumulator housing can be made relatively small, there is a large amount of sheath material of the synthetic core container available, which is preferably wound as evenly as possible so that high strength values are achievable with respect to the kind of windings that are applied. This has no equivalent in the prior art, and causes the production process and thus the resulting bladder accumulators to be cost-effective.

In a preferred embodiment of the method according to the invention provision is made that the bladder body, which is preferably made from an elastomeric material, is provided with an internal vacuum prior to the extrusion of the plastic tube. Due to the vacuum inside the bladder body the elastically yielding material thereof is able to shrink together and make the bladder body slimmer, so that the tube made of soft plastic material, coming from the extruder nozzle and surrounding the bladder body, is able to pass by the same, free of obstruction and with suitable clearance, until the bladder body is fully enveloped inside the plastic tube. In this instance it is possible that the gas inlet valve as one of the media valves is already affixed to the bladder accumulator through vulcanising. Said combination of plastic tube and integrated bladder body with valve is subsequently inserted into a suitable blow moulding tool with its mould halves and through closing of the mould halves, that is, through closing of the moulding tool, the synthetic core container of the bladder accumulator housing is completed, which contains as semi-finished product the bladder body, which is affixed to the synthetic core container via its gas valve that is attached through vulcanising.

In the instance that a valve for liquids is preferably installed into said moulding tool on the side opposite to the gas filling valve, and where the plastic tube is formed into the synthetic core container and the valve for liquids is simultaneously formed into the free end of the tube, the synthetic core container with valves at both ends together with the inserted bladder body can be produced particularly cost-effectively and virtually in a single production step.

Wound subsequently onto the synthetic core container from the outside, and leaving the valve attachment locations including valve components free, is at least one synthetic fibre for the purpose of producing the finished accumulator housing, wherein the preferably used synthetic fibres are carbon fibres. In order to protect the carbon fibre windings they may be embedded in a suitable synthetic resin.

Since the solution according to the invention does no longer require the accumulator bladder, or the bladder body respectively, with its gas filling valve, which is attached through vulcanising, to be inserted through the opening in the housing for the valve for liquids, which usually comes in form of a poppet valve, which requires a relatively large opening cross-section at this location in the accumulator housing, the opening or insertion cross-sections for the accumulator housing valves can be made relatively small even at the liquid end of the respective accumulator. In an average accumulator they may, for example, be in the vicinity of 50 to 60 mm, which leaves a large amount of adjoining sheath surface of the synthetic core container available, providing the opportunity for applying synthetic fibre windings, such as carbon fibre windings.

When applying such windings on a synthetic core container or liner, the placement of the individual fibre bundles or fibre strands, technically called “roving”, is made according to the mathematical condition by Clairaut. This says that the smallest winding angle results from the ratio of maximum diameter and the diameter at the turning point of the respective roving at which its direction changes. If this condition is not met, the respective roving slips off the predeterminable synthetic core container or it leads to unnecessary bunching up of the fibre material in this section, which is then not able to contribute anything to the strength increase.

The “geodesic placements” that deviate to a small extent in practical application from this demand are, however, not detrimental due to the static friction of the individual rovings placed on top of each other. The larger the opening at the respective turning point, formed by the openings for accepting the valves provided in the accumulator housing, the steeper is the winding angle, which leads essentially to a thicker layer structure to achieve the required axial strength for the accumulator housing. If it is possible to reduce the valve insertion opening in the wound accumulator housing according to the invention, the required axial strength in this section may easily be reached with a much reduced layer structure, and the lower number of fibre layers (rovings) on top of each other, which are preferably embedded in surrounding resin, reduces the danger of undesired delamination in this section. Since carbon fibre material is quite expensive, as are other synthetic fibres, it is possible to significantly reduce the production costs for the container through the thinner layer structure that is now possible whilst retaining the same axial strength, so that the container or bladder accumulator as a whole becomes cheaper.

The following explains in greater detail the above-described method according to the invention by way of the embodiment of a bladder accumulator. The only FIGURE depicts a bladder accumulator according to the present invention shown in longitudinal section.

The bladder accumulator shown in the figure and referenced with the number 10 as a whole comprises an accumulator housing 12, which separates two media chambers 16, 18 from each other by means of a bladder body 14. The first media chamber 16 of the bladder accumulator 10 is usually used to hold process gas, such as nitrogen gas, and the second media chamber 18 accepts a liquid, such as hydraulic oil or similar. Said media chambers 16, 18 thus correspond to the usual gas and liquid side respectively of the bladder accumulator 10.

The accumulator housing 12 is constructed in the manner of a composite pressure vessel, comprising an internal synthetic core container 20, which is wound on the outer circumference with a synthetic fibre, which forms a synthetic sheath 22. Instead of using said synthetic materials for the synthetic core container 20 as well as for the synthetic sheath 22, it is possible to use metallic materials, such as aluminium for example, which is easy to mould and is well suited for drawing processes.

The synthetic core container 20 according to the depiction in the figure, technically also called “liner”, is preferably made from polyamide or polyethylene and is made through a blow moulding process or through rotational moulding. Such production methods are commonly known and therefore not explained in detail in this instance.

The invention provides that a gas filling valve 24 is attached to the sheath material of the bladder body 14 through vulcanising, wherein the bladder body 14 is preferably made from an elastomeric, synthetic material, including natural rubber. Said valve 24 not only serves for filling process gas, such as nitrogen gas, into the first media chamber 16 of the bladder body 14; said valve 24 can also be used to remove gas or, if required, to refill again, and wherein valve 24 is also used for gas pressure measurement via a common indicating device that is not shown. The bladder body 14 as well as the gas filling valve 24 are preferably placed together into the usual blow mould, and subsequently a plastic tube is extruded (not shown) over the bladder body 14 with gas filling valve 24 through a corresponding plastic extruder nozzle. The extruded, soft plastic tube with integrated bladder body 14 with valve 24 is then formed through a not-depicted blow moulding tool, wherein the moulding tool with its mould halves has an internal shape which produces the synthetic core container 20 according to the depiction in the figure.

So as not to obstruct the extruding of the plastic tube over the bladder body 14 with valve 24, a vacuum may be applied inside the bladder body 14, preferably via gas filling valve 24, in order to slim down the bladder body 14 so that it does not make contact with the soft plastic material of the plastic tube during the extrusion and forming processes. For the purpose of further simplifying the production process, it is preferred that a valve 26 for liquids, in particular in form of a poppet valve, is inserted into the blow mould tool opposite to the gas filling valve 24, so that also the liquid valve 26 is included by and connected to the plastic tube at its free lower end during the shaping process.

The bladder accumulator 10 shown in the figure has a fluid-conducting connection to a not-depicted hydraulic line of a hydraulic circuit via the liquid supply valve or poppet valve 26. If the pressure at the liquid end is greater than the pressure at the process gas end in form of the media chamber 16, the poppet valve 28, under the additional effect of a compression spring 30 applied thereto, opens up and the second media chamber 18 fills with liquid. Depending on the pressure conditions inside the accumulator, the bladder body 14 with its compressible process gas is compressed, which simultaneously increased the gas pressure inside the first media chamber 16. If liquid is released from the second media chamber 18 of the bladder accumulator 10 through the open liquid valve 26, the elastomeric bladder body 14 is able to expand and potentially apply pressure with its underside on the disk 28 of the poppet valve 26, which then is brought into the closed position against the effect of the compression spring 30, as shown in the figure. The valve disk 28, which is guided longitudinally moveable in a valve housing 32, is provided at its top with a convex surface to make the contact of the accumulator bladder 14 with the top of the disk 28 as gentle as possible.

The valve housing 32 is provided at its outer circumference with an annular groove for retaining a sealing ring 34, which on the outer circumference is in contact with a connection sleeve 36 that protrudes downwards and is part of the completed synthetic core container 20. The liquid valve or poppet valve 26 may be retained in position inside the connection sleeve 36 through a connecting nut 38, for example via a thread connection (not shown).

Since the synthetic core container 20 is rather thin-walled, for example having a wall thickness of 2 mm, an incompressible medium such as water or alcohol is introduced in a predeterminable quantity via the first and/or second media chamber 16, 18 so as to build up a counter-pressure from the inside of the accumulator housing 12 onto the synthetic core container 20, so that windings can be applied to it from the outside without the concern that the liner 20 may buckle or fold to the inside. As a result of the incompressible medium on the inside of the bladder accumulator 10, the thin-walled synthetic core container 20 is retained in position and reinforced in such a way that the windings may be applied without difficulty.

Said liner or synthetic core container 20 is reinforced by fibre windings that are applied at the outside in the manner of a second synthetic sheath 22. The reinforcing windings consist, for example, of fibre reinforcing in the form of rovings, such as carbon fibres, aramit fibres, glass fibres, boron fibres, AL₂O₃ fibres or a blend thereof, which are called hybrid yarns, and which are embedded in a base matrix of duromers (epoxy resins or phenolic resins) or in thermoplastics (PA6, PA12, PP). The fibre-reinforced composite material that forms the supporting shell contains fibre strands (rovings) that cross each other, that can extend in longitudinal and circumferential direction and are embedded in synthetic resin. However, the supporting shell 22 consists preferably of each other crossing fibre strands (rovings) that are inclined in longitudinal and circumferential direction and, in an expedient further development, are inversely inclined along the longitudinal axis of the synthetic core container 20 in relation to each other. The longitudinal and circumferential forces, also seen in axial direction of the accumulator, can thus be optimally retained by the pressure vessel as a whole.

The representation according to the figure demonstrates to those skilled in the art of this field immediately that, in contrast to other solutions in the prior art (DE 10 2006 004 120 A1, DE 10 2011 111 098 A1), the free cross-sectional openings or insertion openings 40, 42 for the gas filling valve 24 and the liquid valve 26 respectively are very small with regard to their diameter, and may be in the vicinity of 50 mm or 60 mm respectively for a 10 litre bladder accumulator 10, for example. This has not been achieved in the prior art so far because, in technically comparable solutions, it was always necessary, with a finished accumulator housing 12, to pull the bladder body 14, or the accumulator bladder respectively, with its gas filling valve 24, attached through vulcanising, via the liquid connection and the inside of the accumulator in the direction of the gas end of the accumulator housing and to attach it there through the respective opening in the accumulator housing. Since, in the solution according to the invention, the synthetic core container 20 yet to be completed has already integrated the bladder body 14 with valve 24 in the forming step, it is no longer necessary to pull it through the inside of the housing at a later stage. Since, as already described, the gas filling valve 24 is already attached, preferably through vulcanising, to the top of the bladder body 14, when closing the moulding tool for the extruded plastic tube under formation of the synthetic core container or liner 20, a kind of annular pinch gap 44 forms at the top of the plastic tube material which, when the plastic tube as liner 20 has hardened, provides a secure attachment location for the gas filling valve 24 in the accumulator housing 12. In a similar manner, a corresponding annular-shaped pinch gap for the liquid valve 26 is formed through the cylindrical connection sleeve 36 of the plastic tube. Nevertheless, it is fundamentally possible to attach the valves 24 and/or 26 later to the finished liner 22 once the accumulator bladder 14 is located inside the accumulator and to affix the valve 24 to bladder body 24 [sic] through an adhesive, for example.

Since, as described, the opening cross-sections for the openings 40, 42 are very small, it is possible to keep the winding angle in this section flat so that even in the section for the valve connection of the synthetic sheath 22 an even and thin layer structure is achieved that does not tend to delaminate but still has the required axial strength. Overall it is possible to achieve a wall thickness for the synthetic sheath 22 of 10 mm for a bladder or accumulator size of 10 litres. The accumulator solution according to the invention permits a direct application of the fibre layers onto the annular pinch gap 44 for the cylindrical connection sleeve 36 for the gas filling valve 24 and for the liquid valve 26 respectively without any intermediary metallic collar components. Since it is possible to completely omit metallic collar components, the accumulator according to the invention is easily constructed but still highly pressure-resistant. 

1. A method for the manufacture of a bladder accumulator (10), which separates two media chambers (16, 18) from each other in an accumulator housing (12) by means of a bladder body (14), comprising at least the following production steps: extrusion of a plastic tube over the bladder body (14); moulding of the plastic tube with integrated bladder body (14) in a moulding tool that corresponds to a predeterminable synthetic core container (20), and winding at least one synthetic fibre from the outside onto the synthetic core container (20) for the purpose of creating the accumulator housing (12).
 2. The method according to claim 1, characterized in that the bladder body (14), which is preferably made from an elastomeric material, is provided with an internal vacuum prior to the extrusion of the plastic tube.
 3. The method according to claim 1, characterized in that the synthetic core container (20), prior to the application of windings from the outside, is stabilised from the inside through the introduction of a preferably incompressible medium to, what will later be, the gas side and/or the liquid side of the bladder accumulator (10).
 4. The method according to claim 1, characterized in that a gas filling valve (24) is attached to the wall material of the bladder body (14) through vulcanising.
 5. The method according to claim 1, characterized in that a valve for liquids (26) is attached to the accumulator housing (12) on the side opposite to the gas supply valve (24), wherein said valve for liquids (26) is operated and brought into a closed position through the expanding bladder body (14) when the accumulator housing (12) is empty.
 6. The method according to claim 1, characterized in that the valve for liquids (26) is inserted into the synthetic core container (20) prior to the application of windings.
 7. The method according to claim 1, characterized in that, when the halves of the moulding tool are closed, the valve for liquids (26) is surrounded by the free end of the plastic tube (36).
 8. The method according to claim 1, characterized in that the diameter of the synthetic core container (20) in the vicinity of where the valve for liquids (26) is incorporated is chosen to be essentially the same, or in the same size range, as the diameter of the synthetic core container (20) in the vicinity of where the gas filling valve (24) is incorporated.
 9. The method according to claim 1, characterized in that the wall thickness of the synthetic core container (20) and the wall thickness of the finished winding is chosen to be essentially the same overall.
 10. A bladder accumulator, produced in particular according to claim 1, which separates two media chambers (16, 18) from each other in an accumulator housing (12) by means of a bladder body (14), wherein each of said media chambers (16, 18) is provided with a media valve (24, 26), characterized in that the mounting diameter of both media valves (24, 26) in accumulator housing (12) correspond to each other or are at least in the same size range, and are chosen to have preferably small opening diameters (40, 42). 